The present invention relates to a method of establishing the carrier-to-interference ratio in a radio communication system. The invention also relates to a communication device carrying out such a method.
In a communication system comprising a number of radio base stations and mobile communication devices communication is established between a mobile communication device and a base station by means of a radio channel, which is also referred to as a transmission link. Channel access is achieved by, for example, frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) or combinations thereof.
When a transmission link has been established between a base station and a mobile communication device it is important to be able to measure the quality of the link. Several methods for doing this exist. According to one method the received signal strength indication, RSSI or RXLEV, is used. It is then simply assumed that the received signal strength indication is correlated to the quality of the transmission link. However, the received signal strength includes both the carrier and the interference and, hence, is not a good measure of the quality of the transmission link. According to a better method the bit error rate, RXQUAL, is used as an estimate of the quality of the transmission link, i.e. a small error bit rate corresponds to a high quality transmission link and vice versa. However, due to the data compression carried out before information is transmitted over a transmission link the bit error rate is quantified to a small amount of levels, e.g. eight levels in the standard of GSM (Global System for Mobile communication). Consequently, this method only gives a rough indication of the quality of the transmission link. Furthermore, the channel coding scheme used may not allow all bit errors to be detected, and the bit error rate will then only be an estimate of the quality of the transmission link. This is true, for example, for the channel coding scheme used in GSM. Moreover, the quality of the estimate is normally better for small bit error rates than for large bit error rates.
The ratio between the received signal strength of the carrier and the received signal strength of the interference, C/I, is a better measure of the quality of the transmission link. This parameter is used for designing cellular radio communication systems, for example during development of channel coding schemes and speech coder algorithms. The parameter is also important during cell-planning, i.e. determining the sites of the base stations and measuring power levels at different locations in the system. Furthermore, the communication system will gain valuable information if C/I can be measured by the base station or the mobile communication device when the communication system is in use. For example, the measured C/I information can make it possible for the communication system to perform hand-off to a transmission link of good quality at an advantageous opportunity.
Stand-alone C/I measurement equipment is known in the art. However, since they employ high quality receivers and advanced filtering techniques they are expensive and, hence, cannot be used in ordinary mobile communication devices.
Several methods are available for estimating the C/I parameter.
One method is disclosed in SE-A-469580. The method, which involves at least two base stations, can only be used in up-link, i.e. the transmission link from a mobile communication device to a base station. The signal strength of the carrier is measured by the base station with which the mobile communication device has established a communication link. At the same time the second base station measures the signal strength from the same mobile communication device. This signal strength is then used as an estimate of the interference and an estimate of the C/I parameter is calculated. A major disadvantage of this method is that it is necessary for the two base stations to cooperate and that only an estimate of the C/I in up-link can be established.
Another method is disclosed in JP-A-08/182042. The signal strength of the carrier is measured on the established communication link and the interference is estimated by a measuring the signal strength of a currently empty channel. However, the accuracy of the estimate of C/I provided by this method is poor since the interference experienced on the established communication link may be quite different from the interference measured on the empty channel, i.e. at a different radio frequency.
The method disclosed in U.S. Pat. No. 5,583,886 concerns CDMA systems. Each of a plurality of transmitters transmits one of a plurality of signals which are known to the receiver. A plurality of channel responses are estimated by measuring the known signals at the receiver. The channel responses are then used to determine the C/I parameter. A drawback with this method is that it is required that the receiver knows the plurality of signals and the timing thereof. Furthermore, the transmission of known signals occupies resources of the system. WO-A-92/17953 refers to the measurement of the carrier-to-interference of a selected target channel before hand-off. The mobile communication device measures the received signal strength indication of the selected target channel when the channel is transmitting and not transmitting. The measurements are transferred to the source base station which determines the C/I parameter from the measurements and decides whether hand-off should be carried out to the selected target channel. The disadvantages of this method are that the C/I is only measured on target channels and not on the source channel and that the measurements are only performed occasionally. The method does not provide for continuous measurements.
It is an object of the present invention to provide a method of measuring the C/I parameter which overcomes or alleviates the above mentioned problems.
The invention is also directed to a communication device incorporating the method of measuring the C/I parameter.
According to one aspect of the present invention there is provided a method of estimating the carrier-to-interference ratio in a radio communication system comprising the steps of selecting a radio communication channel allowing discontinuous transmission, measuring the received signal strength in the communication channel within periods of time when transmission is discontinued, measuring the received signal strength in the communication channel within periods of time with transmission and calculating an estimate of the carrier-to-interference ratio of the channel by means of the signal strengths measured in the previous steps.
According to a further aspect of the present invention there is provided a radio communication device comprising a radio receiver adapted for receiving a signal picked up by an antenna at a selected channel, means for measuring the received signal strength at the selected channel, controller means for establishing periods of time corresponding to transmission and discontinued transmission, respectively, within the selected channel and calculation means for estimating the carrier-to-interference ratio, C/I, from the measured signal strength within periods of discontinued transmission and the measured signal strength within periods of transmission.
The method and device achieve the advantages that the carrier-to-interference ratio can be estimated continuously and that it can be estimated without affecting the communication on a channel.
These advantages are achieved by measuring the signal strengths at a channel within periods of transmission and discontinued transmission. This can be done continuously without affecting the communication on the channel.
Furthermore, the ratio can be measured and estimated using a relatively simple hardware and software solution thereby allowing the method to be implemented in a small physical application, such as a portable communication device.
This advantage is achieved by the possibility of using a conventional RSSI detector.
According to a further aspect of the present invention there is provided a method of controlling channel selection within a radio communication system having at least two communication devices comprising the steps of establishing a communication channel between a first and a second communication device, providing a list of frequencies which may be used for a communication channel and for each frequency establishing whether discontinued transmission is activated and if so measuring the received signal strength at the frequency within periods of time when transmission is discontinued, measuring the received signal strength at the frequency within periods of time with transmission, and calculating an estimate of the carrier-to-interference ratio at the frequency by means of the signal strengths measured in the two previous steps, ranking the frequencies of the list according to the corresponding estimated levels of C/I, communicating information representing at least the frequency having the greatest C/I to at least the first and the second communication devices, and setting up a communication channel between the first and the second communication devices using at least the frequency having the greatest C/I.
This method achieves the advantage that a communication channel having the greatest C/I ratio can be set up without having to disturb communication on the channel when the C/I ratio is measured and estimated.
This advantage is achieved by measuring the signal strengths at a channel within periods of transmission and discontinued transmission. This c an be done without affecting the communication on the channel.
Preferably, information representing at least two of the frequencies having the greatest C/I is communicated to the at least first and second communication devices and the communication channel set up between the at least first and second communication devices uses at least the two frequencies having the greatest C/I in a frequency hopping scheme.
The advantage of this is that a frequency hopping scheme can be establish ed based on estimated C/I ratio s without the measurements needed for estimating these C/I ratios affecting the communication at the corresponding frequencies.
This advantage is achieved by measuring the signal strengths at the frequencies within periods of transmission and discontinued transmission. This can be done without affecting the communication at the frequencies.
Preferably C/I is estimated by calculating according to the formula (C/I)dB≈((C+I)/I)dB=(C+I*)xe2x88x92(I*) where the measured signal strength, in dBm, corresponding to periods of discontinued transmission is labeled (I*) and the measured signal strength, in dBm, corresponding to periods of transmission is labeled (C+I*)
The advantage of this is that the calculation needed for estimating C/I in dB is very simple and, hence, easy and cheap to implement in a device.
This advantage is achieved by estimating C/I by a simple subtraction.
Preferably, if C/I is estimated by calculating according to the formula (C/I)dB≈((C+I)/I)dB=(C+I*)xe2x88x92(I*) the thus estimated value of C/I is corrected according to a correction function.
The advantage of this is that the error ensuing at small values of C/Ixe2x80x94due to the fact that the signal strength of the carrier plus the disturbances are measured at the same timexe2x80x94can be compensated.
This advantage is achieved by the correction function.
Preferably, the method comprises the steps of reading out from a memory a pre-stored correction value which corresponds to the estimated C/I value and correcting the estimated C/I value by using the read out correction value according to a correction function.
Preferably, the device comprises a memory for holding at least one pre-stored correction value, each pre-stored correction value being associated with a corresponding C/I value, read out means for reading out a correction value from the memory and providing the read out correction value to the calculation means. Furthermore, the calculation means is adapted to correct the estimated C/I by means of the read out correction value according to a correction function.
The advantage of this method and this device is that the correction value or values can be pre-calculated and pre-stored and, hence, quickly retrieved when needed.
This advantage is achieved by pre-calculating and pre-storing the correction value or values in the memory.
According to a further aspect of the present invention there is provided a method of operating a communication device having a radio receiver and a radio transmitter, the method comprising the steps of selecting a radio communication channel allowing discontinuous transmission, measuring the received signal strength in the communication channel within periods of time when transmission is discontinued, measuring the received signal strength in the communication channel within periods of time with transmission, and transmitting information representing the signal strengths measured in the previous steps by means of the transmitter.
According to a further aspect of the present invention there is provided a radio communication device comprising a radio receiver adapted for receiving a signal picked up by an antenna at a selected channel, means for measuring the received signal strength at the selected channel, controller means for establishing periods of time corresponding to transmission and discontinued transmission, respectively, within the selected channel, and a transmitter for transmitting information representing the measured signal strength within periods of discontinued transmission and for transmitting information representing the measured signal strength within periods of transmission from the communication device.
This method and device achieve the advantage that though the measurements needed for establishing the C/I ratio are performed by the communication device, the latter does not need to be equipped with the calculation means needed for calculating the estimate of the C/I ratio.
This advantage is achieved by transmitting the measured signal strengths from the communication device.